Here's the thought process of the time.
First off, realize that farmers don't grow food for the love of it, they do it to support their families.
If farmers don't make enough selling their crops/livestock, their farms dissapear. You get this happening to enough farms; it hits critical mass, and you have a famine.
This was put into place to hopefully avoid having the problem hit critical mass.
The tax on ag processors was a way to pay for things in such a way that it could be continued. Just paying out for crops probably wouldn't be sustainable at all, even though that makes better sense for the time.
In recent memory, it's always been the policy of the government that food be cheap enough that almost anyone can afford it. It's almost always been the policy that food is too important to trust free market forces to set prices for.
In reality I think this "cheap food policy" has helped keep many from starving, but has also pushed many into unhealthy food choices, so it's giving with one hand, and smacking with the other.
I'm not an über-economist, other that what I studied in HS and the bit of college I took. In honesty, I can defend the polcy as well as attack it, and can make either side make sense in my head.